Worldwide, cardiovascular diseases are among the leading causes of mortality and morbidity with ever-increasing prevalence. Early targeted initiation of preventive measures would be of great benefit and can provide a major opportunity in reducing mortality and morbidity. To this end, accurate identification of individuals who are still asymptomatic but at elevated risk is essential. However, traditional risk assessment fails to recognize a substantial proportion of patients at high risk while a large proportion of individuals are classified as having intermediate risk, leaving patient management uncertain. Additional strategies to further refine risk assessment are therefore highly needed. To this end, the inventors have evaluated the role of novel lipidomic biomarkers as diagnostic tool for atherosclerosis. These new biomarkers can be used to identify an individual at elevated risk for CVD events such as angina pectoris, myocardial infarction, stroke and cardiovascular death.
Plasma or serum total cholesterol, LDL-cholesterol or HDL-cholesterol concentrations have been used as gold standard biomarkers for CVD/CAD risk prediction. However, a number of coronary artery disease (CAD) or acute myocardial infarction (AMI) patients have LDL-C levels within the recommended range suggesting the need for additional diagnostic measures of the residual risk. It is evident from earlier large scale population studies that these measurements associate with the CAD risk and CAD endpoints such as AMI or cardiovascular death. Therefore, preventive treatment strategies have so far been addressed to lower LDL-C concentrations (mainly by statin treatment) and more recently also attempts to raise HDL-C have been made (e.g. by CETP-inhibitors). On the other hand, it has also been observed that one half of the AMI patients actually do have normal LDL cholesterol levels and that there is a substantial residual risk in statin treated patients despite a LDL-C lowering. Furthermore, recent publications have demonstrated that plasma levels of apolipoprotein B (apoB), the main surface protein on LDL particles, and LDL-C, the amount of cholesterol in those particles, are correlated and, considered separately, as positive risk factors. Plasma levels of apolipoprotein A1, the main surface protein on HDL particles, and HDL-C, the amount of cholesterol in those particles, are also correlated with each other and, considered separately, as negative risk factors. Importantly, for a given usual apoB, lower LDL-C has been observed to associate with a higher risk of AMI supporting the view that, on average, LDL particles with low cholesterol content per particle (small, dense LDL particles) are particularly hazardous. Thus, it seems possible that LDL-C associates directly with the more dangerous molecules carried by LDL-particle and that LDL-C is only an indirect measurement of the risk. Therefore, it is of importance to search for molecules e.g. lipid species that play an active role in the CAD development.
Lipid metabolite imbalance is a probable cause of dyslipidemia and the ensuing atherosclerosis manifested in its gravest form as the vulnerable atherosclerotic plaque. Atherosclerotic plaques are complex molecular formations that contain numerous lipids. However, there are other factors than lipid rich plaques or LDL cholesterol that make lipids an attractive group of molecules for CVD studies. Lipids are tightly regulated which makes Lipidomic data robust and informative on the current state of the studied organism. Also, lipids are one of the culmination points of a biological system, more the true outcome than the predictor. Combining Lipidomic data with appropriate biobanked clinical material presents a good opportunity for biomarker discovery. Moreover, lipidomics can be used as a gauge of efficacy and safety in drug development and evolving theragnostics. Lipidomic biomarkers are prime candidates for true companion diagnostics in the CVD area and present many opportunities for improved translational medicine as well.
The plaque building blocks and lipoprotein components that are thought to traffic the lipids to the site of lesion formation can now be resolved with Lipidomic studies correlating lipid structure and composition to function and thereby disease pathogenesis. While the number of lipid mediators in the human body is overwhelming their identification and quantification is facilitated by the advances in mass spectrometry and lipid biochemistry, which today enable the simultaneous high throughput identification and quantification of hundreds of molecular lipid species in several lipid classes (Ejsing C S, et al: Global analysis of the yeast lipidome by quantitative shotgun mass spectrometry. Proc Natl Acad Sci USA 2009, 106:2136-2141; Stahlman M, et al: High-throughput shotgun lipidomics by quadrupole time-of-flight mass spectrometry. J Chromatogr B Analyt Technol Biomed Life Sci 2009 Hiukka A, et al: ApoCIII-enriched LDL in type 2 diabetes displays altered lipid composition, increased susceptibility for sphingomyelinase, and increased binding to biglycan. Diabetes 2009, 58:2018-2026; Linden D, et al: Liver-directed overexpression of mitochondrial glycerol-3-phosphate acyltransferase results in hepatic steatosis, increased triacylglycerol secretion and reduced fatty acid oxidation. FASEB J2006, 20:434-443.) collectively referred to as the lipidome. Lipidomic studies identify lipid cellular distribution and describe their biochemical mechanisms, interactions and dynamics. Importantly, lipidomics quantifies the exact chemical composition of lipidomes (Han X, Gross R W: Global analyses of cellular lipidomes directly from crude extracts of biological samples by ESI mass spectrometry: a bridge to lipidomics. J Lipid Res 2003, 44:1071-1079).
Due to both high sensitivity and selectivity of lipidomics, even the smallest sample amounts can be analyzed today. The bulk of the lipid data in the art today presents lipids in a sum composition format, i.e. phosphatidylcholine (PC) 34:1 (Brugger B, et al: Quantitative analysis of biological membrane lipids at the low picomole level by nano-electrospray ionization tandem mass spectrometry. Proc Natl Acad Sci USA 1997, 94:2339-2344) where the molecular lipid and the attached fatty acid tails remain unidentified. The identification of molecular lipid species, e.g., PC 16:0/18:1 (Ekroos K, et al: Charting molecular composition of phosphatidylcholines by fatty acid scanning and ion trap MS3 fragmentation. J Lipid Res 2003, 44:2181-2192) is the main feature of advanced lipidomics, which delivers highly resolved molecular lipid species rather than summed fatty acid information. For example, the information of the type of fatty acids and their positions of attachment to the glycerol backbone making up the particular PC molecule is revealed. There are conventional techniques such as thin-layer chromatography combined with gas chromatography but they not only require considerably larger sample amounts and laborious sample preparation, but they do not deliver the molecular lipid species. Despite multiple mass spectrometry techniques capable of characterizing lipid entities, most of them are still unable to deliver reliable high-quality quantitative data in terms of absolute or close-to absolute concentrations. In the context of the present invention, electrospray ionization mass spectrometry based lipidomics is the preferred technology and can utilize both shotgun and targeted lipidomics for exhaustive deciphering and precise quantification of molecular lipidomes. The superior quality and specificity of shotgun and targeted lipidomics will meet stringent regulatory standards, such as good laboratory practice guidelines (GLP) when set-up in the proper environment. Using these technologies quantification of up to two thousand molecular lipids is possible even in a high throughput format.
Lipidomics is a tool for differentiating patients based on their molecular lipid profiles. Personalized medicine and diagnostics enabled by lipidomics will facilitate the mission of the right individual receiving the right drug at the right time and dose. Several works employing analytes consisting of lipids, proteins and hydrophilic molecules among many others have been conducted to meet the needs of personalized medicine. Recently, non-hypothesis-driven metabolomic screenings have been used to identify novel CVD biomarkers.
For example, WO2004/038381 discloses a method for metabolomically facilitating the diagnosis of a disease state of a subject, or for predicting whether a subject is predisposed to having a disease state wherein the small molecule profile from a subject is obtained and compared to a standard small molecule profile.
Additionally, Zhang et al. utilized an ultra fast liquid chromatography coupled with IT-TOF mass spectrometry (UFLC/MS-IT-TOF) to study plasma and urine metabolic profiles of atherosclerosis rats (Zhang F, et al: Metabonomics study of atherosclerosis rats by ultra fast liquid chromatography coupled with ion trap-time offlight mass spectrometry. Talanta 2009, 79:836-844). Their observations suggest that abnormal metabolism of phenylalanine, tryptophan, bile acids, and amino acids might be related to atherosclerosis development. Furthermore, Zha et al. identified a metabolic fingerprint of twenty-one compounds in hamsters that could be a potential marker for the development of atherosclerosis (Zha W, et al: Metabonomic characterization of early atherosclerosis in hamsters with induced cholesterol. Biomarkers 2009, 14:372-380).
More specifically, US2008/0085939 relates to the use of specific sphingolipids, especially phytosphingosine, sphingosine and/or sphinganine for prevention and/or treating inflammatory processes associated with atherosclerosis. This is based on the observation that sphingolipids have an anti-inflammatory effect. Similarly, WO2008/148857 discloses a method to assess the risk of cardiovascular disease in a patient (including atherosclerosis) by isolating the HDL fraction and sub-fraction from a blood sample of the patient. The components of the HDL fraction or sub-fraction to be measured were Sphingosine-1-Phosphate (SIP), sphingomyelin (SM) and Apolipoprotein A-I (apoA-1).
WO2008/11943 further discloses markers for detecting coronary artery disease that can indicate a patient at risk of having or developing coronary artery disease. These include 15 “first-choice” molecules which were: C18:3 Cholesterol ester, C32:1 Phosphatidylcholine, Alanine, Lipid (mainly VLDL), Lysine, Hexadecanoic acid, C36:2 Phosphatidylcholine, Formate, C32:2 Phosphatidylcholine, C18:2 (Linoleic Acid), Cholesterol, C 18:2 Lyso-phosphatidylcholine, C36:3 Phosphatidylcholine, C34:4 Phosphatidylcholine and C34:3 Phosphatidylcholine. Furthermore, US2007/0099242 describes a method to determine if a subject is at risk to develop, or is suffering from cardiovascular disease. The method involves determining a change in the amount of a biomarker in the biological sample or HDL sub-fraction thereof, compared to a control sample, wherein the biomarker is at least one of Apolipoprotein C-IV (“ApoC-IV”), Paraoxonase 1 (“PON-1”), Complement Factor 3 (“C3”), Apolipoprotein A-IV (“ApoA-IV”), Apolipoprotein E (“ApoE”), Apolipoprotein LI (“ApoL1”), Complement Factor C4 (“C4”), Complement Factor C4B1 (“C4B1”), Histone H2A, Apolipoprotein C-II (“ApoC-II”), Apolipoprotein M (“ApoM”), Vitronectin, Haptoglobin-related Protein and Clusterin. The document also discloses a method for detecting the presence of one or more atherosclerotic lesions wherein a change in the amount of a biomarker in the biological sample or HDL sub-fraction thereof is detected, compared to a control sample and wherein the biomarker is selected from PON-1, C3, C4, ApoE, ApoM and C4B1. All biomarkers mentioned in this document are protein or lipoprotein biomarkers.
From previous work it cannot be extrapolated that lipid analysis will yield by default a CVD biomarker. Earlier analytical attempts to find specific biomarkers, which have also included some lipid analytes did not generate novel lipid biomarker findings of clinical value. The present invention identifies CVD biomarkers by absolute, or close to absolute, quantification of defined molecular lipid species instead of profiling multiple analytes. Importantly, while many of the existing biomarker candidates are composite fingerprints of multiple factors, the lipidomics approach herein shows value already at a level of single species or ratios thereof. The present invention herein is the first one where the lipid biomarker concentrations have been measured and quantified, therefore adding more accuracy to the findings and enabling accurate usage of the lipid based biomarker. The lipid biomarkers herein have not been linked to CVD risk prediction in the above publications. Another layer of accuracy was reached through a careful patient selection as it is important to take into account those factors which may affect the measured lipid concentrations. Thus, we excluded statin users, matched cases and controls for apoB levels (major lipid carrier particle in blood) and included only males. Unlike the previous efforts described above we used specific targeted platforms on a singular technology set-up to analyze lipid species in particular.
The technology and the way it was applied in the context of the inventive teaching presented herein is set apart from similar efforts in the field inter alia due to the following criteria. In sample preparation, samples are strictly controlled and treated identically to avoid potential artifacts that could arise from improper handling. In connection with the present invention, samples were carefully thawed slowly on ice and directly thereafter subjected to a custom made automated lipid extraction which possesses currently the highest precision in liquid handling, therefore minimizing potential errors. Furthermore, sample freeze-thaw cycles were strictly controlled since this can dramatically affect the lipid stabilities. The automated lipid extraction is based on the method by Folch and colleagues (Folch J, et al: A simple method for the isolation and purification of total lipids from animal tissues. J Biol Chem 1957, 226(1):497-509) which uses chloroform and methanol. This method is preferred when a wide range, from polar to non-polar, of lipid classes are to be extracted with optimal recoveries thus preventing the loss of lipid species. Lipid class specific non-endogenous lipids, when applicable, were used as internal standards to gain highest precision in identification (minimizing false positives) and quantification of monitored molecular lipid species. In this way absolute or semi-absolute amounts of endogenous molecular lipids were determined with the highest precision that can be achieved with today's technologies. The endogenous lipids and respective standards were monitored at the molecular lipid level. In this way, not only false positive identifications were minimized, but molecular lipids could be precisely determined and quantified. Analysis quality was strictly controlled using a novel quality control system. This was mainly controlled by multiple internal standards (IS), external standards (ES), IS/ES ratios, and instrument control samples. By stringently controlling these components, technical and biological outliers were readily identified and rejected from further analysis. To obtain best precision in sensitivity, selectivity and quantification for each molecular lipid different targeted platforms were used. Some lipids are best analyzed using high performance liquid chromatography (HPLC) or ultra high performance liquid chromatography (UHPLC) combined with mass spectrometry based multiple reaction monitoring (MRM) whereas others are best analyzed by direct infusion in combination with mass spectrometry based precursor ion scanning and neutral loss scanning techniques.